"git merge-base" names its internal workhorse helper function
"get_merge_bases_many_0()", which takes one "can we get away without
clearing the object->flags bits because we know we are the last
caller?" parameter. Make the parameter into a flags word to make it
extensible and rename it to get_merge_bases_opt(). Use it to turn
get_merge_bases_many_dirty() wrapper into a C-preprocessor macro.